Microfluidics, which concerns the control of the flow and transfer of a very small amount of fluid, is essential for driving an apparatus for diagnosing and analyzing a sample, which may be executed using various driving methods such as a pressure-driven method using an applied pressure to a fluid injection portion; an electrophoretic method using an applied voltage across a microchannel; an electroosmotic method; and a capillary flow method using capillary force.
A typical example of a microfluidic device driven using a pressure-driven method is illustrated in U.S. Pat. No. 6,296,020, in which the cross-sectional area of a channel and the hydrophobicity of the channel are controlled with a passive valve installed in a hydrophobic fluidic circuit device. In addition, U.S. Pat. No. 6,637,463 discloses a microfluidic device in which channels having pressure gradients have been designed so that a fluid is uniformly distributed through the channels.
The capillary flow method, in particular, which uses capillary force spontaneously occurring in microchannels is advantageous because a very small amount of a fluid moves spontaneously and instantly along specific channels without the use of an additional driving means. Hence, extensive studies of microfluidic systems using such capillary flow method have been recently conducted. For example, U.S. Pat. No. 6,271,040 discloses a diagnostic biochip in which a sample is transferred using only the naturally-occurring capillary flow in microchannels without the use of a porous material, and the sample transferred in such a way was allowed to react with the biochips to detect a specific component in the sample. Also, U.S. Pat. No. 6,113,855 discloses a diagnostic apparatus in which hexagonal micro-pillars are appropriately arranged to generate capillary force for transferring a sample through the space between the pillars.
In order to achieve a satisfactory flow of a fluid in the conventional microfluidic device using the capillary flow method, the surface wettability of the capillary wall must be good. In case of a conventional plastic microfluidic device, such surface wettability of the plastic is unacceptably low, and to improve the poor surface wettability, a treatment, e.g., corona, surface coating and plasma treatments, has been conventionally used. For example, a method of roughening the inner surface of a microfluidic channel to enhance a fluid flow rate has been reported in WO 2007/075287.
However, the above methods for improving the wettability makes it difficult to achieve mass production of microfluidic devices, and they may also cause processing problems such as a need for the use of additional devices to carry out additional tasks. Further, because the effects of these treatment methods may deteriorate over a large period of use, it is difficult to maintain a constant, stable flow of a fluid.